Dummy core restrict resin process and structure

ABSTRACT

A printed circuit board (PCB) has multiple layers, where select portions of inner layer circuitry, referred to as inner core circuitry, are covered by a coverlay material and the covered inner core circuitry is exposed from the remaining layers of the PCB. The PCB having covered inner core circuitry is formed using a dummy core plus coverlay process. The select inner core circuitry is part of an inner core. The inner core corresponding to the covered inner core circuitry forms a flexible PCB portion. The flexible PCB portion is an extension of the remaining adjacent multiple layer PCB. The remaining portion of the multiple layer PCB is rigid. The inner core is common to both the flexible PCB portion and the remaining rigid PCB portion.

CROSS-REFERENCE TO RELATED APPLICATION(S)

This application claims priority under 35 U.S.C. § 119(a)-(d) of theChinese Patent Application No: 201610090048.5, filed Feb. 17, 2016 andtitled, “DUMMY CORE RESTRICT RESIN PROCESS AND STRUCTURE,” which ishereby incorporated by reference in its entirety for all purposes.

FIELD OF THE INVENTION

The present invention is generally directed to printed circuit boards.More specifically, the present invention is directed to printed circuitboards having select exposure of inner layer circuitry.

BACKGROUND OF THE INVENTION

A printed circuit board (PCB) mechanically supports and electricallyconnects electronic components using conductive traces, pads and otherfeatures etched from electrically conductive sheets, such as coppersheets, laminated onto a non-conductive substrate. Multi-layered printedcircuit boards are formed by stacking and laminating multiple suchetched conductive sheet/non-conductive substrate. Conductors ondifferent layers are interconnected with plated-through holes calledvias.

A printed circuit board includes a plurality of stacked layers, thelayers made of alternating non-conductive layers and conductive layers.The non-conductive layers can be made of prepreg or base material thatis part of a core structure, or simply core. Prepreg is a fibrousreinforcement material impregnated or coated with a thermosetting resinbinder, and consolidated and cured to an intermediate stage semi-solidproduct. Prepreg is used as an adhesive layer to bond discrete layers ofmultilayer PCB construction, where a multilayer PCB consists ofalternative layers of conductors and base materials bonded together,including at least one internal conductive layer. A base material is anorganic or inorganic material used to support a pattern of conductormaterial. A core is a metal clad base material where the base materialhas integral metal conductor material on one or both sides. A laminatedstack is formed by stacking multiple core structures with interveningprepreg and then laminating the stack. A via is then formed by drillinga hole through the laminated stack and plating the wall of the hole withelectrically conductive material, such as copper. The resulting platinginterconnects the conductive layers in the laminated stack.

In some applications, it is desirable to form part of the printedcircuit board with a reduced number of layers, which are flexible, toform a flexible portion that is bendable yet remains interconnected toother rigid portions of the printed circuit board, thereby forming arigid-flexible printed circuit board. Current process flow is to pre-cutprepreg at a desired flexible portion and then control resin squeeze outduring the lamination process. This process flow has disadvantages suchas high cost of low flow prepreg, limited supply of low flow prepreg anddifficulty in controlling resin squeeze out. Additionally, laminationaccessories such as release film and conformal film are needed whichalso add cost. Release film provides a separation between a surfacecopper layer (conducting layer) in the lamination stack and theconformal film. Conformal film is a thermoplastic layer which softensunder lamination temperature and conforms to the area with prepregpre-cut. This reduces prepreg resin flowing into the flexible portion.However, resin can still flow into the rigid-flexible boundary randomly,resulting in an irregular rigid-flexible boundary. Such an irregularboundary forms a serrated surface that cuts against the flexibleportion. Further, lamination under high pressure and the impact ofconformal film can result in increased panel distortion and it isdifficult to achieve flat surface for fine line etching or evendielectric thickness across panel to control impedance. A panel hererefers to the finished product of the stack of laminate and prepregafter lamination. In order to solve these issues, a new manufacturingprocess for rigid-flex printed circuit boards is needed.

SUMMARY OF THE INVENTION

Embodiments are directed to a PCB having multiple layers, where selectportions of inner layer circuitry, referred to as inner core circuitry,are covered by a coverlay material and the coverlay and inner corecircuitry are exposed from the remaining layers of the PCB to form aflexible PCB portion. The PCB having an exposed coverlay and inner corecircuitry is formed using a dummy core plus coverlay process. The selectinner core circuitry is part of an inner core. During manufacturing ofthe PCB, a coverlay is applied over the select inner core circuitry anda dummy core is applied over the coverlay. The coverlay and the dummycore protect the select inner core circuitry during subsequent processsteps and also enable exposure of the coverlay and select inner corecircuitry as described in detail below. The flexible PCB portion is anextension of the remaining adjacent multiple layer PCB. The remainingportion of the multiple layer PCB is rigid, referred to as a rigid PCBportion. The inner core is a layer(s) of the PCB and is therefore commonto both the flexible PCB portion and the remaining rigid PCB portion.The flexible PCB portion can be formed as an interior portion of the PCBsuch that a rigid PCB portion is coupled to either end of the flexiblePCB portion.

In an aspect, a printed circuit board is disclosed. The printed circuitboard includes a rigid printed circuit board portion and a flexibleprinted circuit board portion. The rigid printed circuit board portioncomprises a laminated stack of a plurality of non-conducting layers anda plurality of conductive layers, wherein the laminated stack furthercomprises a first portion of an inner core structure. The flexibleprinted circuit board portion comprises a second portion of the innercore structure, wherein the inner core structure is a continuousstructure that extends through both the rigid printed circuit boardportion and the flexible printed circuit board portion. The secondportion of the inner core structure comprises inner core circuitry andexposed coverlay material covering the inner core circuitry. In someembodiments, each of the conductive layers is pattern etched. In someembodiments, the printed circuit board further comprises one or moreplated through hole vias in the rigid printed circuit board portion. Insome embodiments, the rigid printed circuit board portion comprises afirst rigid printed circuit board portion, further wherein the printedcircuit board further comprises a second rigid printed circuit boardportion comprising a second laminated stack of a plurality ofnon-conducting layers and a plurality of conductive layers, wherein thesecond laminated stack further comprises a third portion of the innercore structure, further wherein the flexible printed circuit boardportion is coupled between the first rigid printed circuit board portionand the second rigid printed circuit board portion. In some embodiments,the inner core structure comprises an inner core non-conductive layerhaving a first surface and a first conductive layer positioned on thefirst surface of the inner core non-conductive layer. In someembodiments, the first conductive layer of the inner core structurecomprises the inner core circuitry in the second portion of the innercore structure. In some embodiments, the inner core non-conductive layerhas a second surface opposing the first surface, further wherein theinner core structure further comprises a second conductive layerpositioned on the second surface of the inner core non-conductive layer.In some embodiments, the second conductive layer of the inner corestructure comprises the inner core circuitry in the second portion ofthe inner core structure. In some embodiments, the inner corenon-conductive layer comprises polyimide. In some embodiments, thecoverlay material comprises a combination of polyimide and adhesive.

In another aspect, a printed circuit board set form is disclosed. Theprinted circuit board set form comprises a plurality of printed circuitboards and breakaway substrate. The plurality of printed circuit boardsare aligned within a common plane, wherein each printed circuit board ismechanically connected by a common substrate. Each printed circuit boardcomprises a rigid printed circuit board portion and a flexible printedcircuit board portion. The rigid printed circuit board portion comprisesa laminated stack of a plurality of non-conducting layers and aplurality of conductive layers, wherein the laminated stack furthercomprises a first portion of an inner core structure. The flexibleprinted circuit board portion comprises a second portion of the innercore structure, wherein the inner core structure is a continuousstructure that extends through both the rigid printed circuit boardportion and the flexible printed circuit board portion. The secondportion of the inner core structure comprises inner core circuitry andexposed coverlay material covering the inner core circuitry. Thebreakaway substrate is aligned within the common plane and mechanicallyconnected around a perimeter of the connected plurality of printedcircuit boards, wherein the breakaway substrate includes a dummy coreportion. In some embodiments, the breakaway substrate provides lateralstructural stability to the connected plurality of printed circuitboards. In some embodiments, the plurality of printed circuit boards areelectrically isolated from each other.

In yet another aspect, a method of manufacturing a printed circuit boardis disclosed. The method comprises forming an inner core structurehaving an inner core circuitry on at least one surface of the inner corestructure and applying a coverlay material over the inner corecircuitry. The method also comprises forming a printed circuit boardstack up, wherein the printed circuit board stack up comprises the innercore structure, a dummy core, one or more non-conductive layers and oneor more conductive layers, wherein the dummy core is stacked on thecoverlay material. The method also comprises laminating the printedcircuit board stack up, thereby forming a laminated stack. The methodalso comprises forming a depth controlled rout from a surface of thelaminated stack to the dummy core and around a perimeter of the dummycore, wherein a portion of the laminated stack within the perimeter ofthe rout and to a depth including the dummy core forms a laminated stackcap. The method also comprises removing the laminated stack cap, therebyexposing the coverlay material and forming a flexible portion of theprinted circuit board. In some embodiments, the perimeter of the dummycore corresponds to a perimeter of the inner core circuitry. In someembodiments, the method also comprises forming the dummy core, whereinthe dummy core comprises a non-conductive layer and a conductive layer.In some embodiments, the dummy core is stacked on the coverlay materialsuch that the conductive layer of the dummy core contacts the coverlaymaterial. In some embodiments, the method also comprises forming atleast one plated through hole via in the laminated stack prior toforming the depth controlled rout, wherein the at least one platedthrough hole via is not aligned within the inner core circuitry. In someembodiments, the method also comprises pattern etching the conductivelayers in the laminated stack prior to forming the printed circuit boardstack up. In some embodiments, forming the inner core structurecomprises applying a first conductive layer on a first surface of anon-conductive layer and applying a second conductive layer on a secondsurface of the non-conductive layer. In some embodiments, the firstconductive layer is pattern etched and the second conductive layer ispattern etched. In some embodiments, the one or more non-conductivelayers comprise one or more regular flow prepreg layers. In someembodiments, laminating the printed circuit board stack up comprisesapplying a standard lamination pressure less than about 450 psi. In someembodiments, a remaining portion of the laminated stack outside theperimeter of the rout forms a rigid portion of the printed circuitboard, wherein an exposed outer surface of the laminated stack is smoothand non-rippled due to laminating the printed circuit board stack upusing regular lamination pressure and the inclusion of regular flowprepreg.

In yet another aspect, another printed circuit board is disclosed. Theprinted circuit board comprises a rigid printed circuit board portionand a flexible printed circuit board portion. The rigid printed circuitboard portion comprises a laminated stack of a plurality ofnon-conducting layers and a plurality of conductive layers. Theplurality of non-conducting layers comprises a plurality of regular flowprepreg layers. An exposed outer surface of the laminated stack issmooth and non-rippled. The laminated stack further comprises a firstportion of an inner core structure. The flexible printed circuit boardportion comprises a second portion of the inner core structure. Theinner core structure is a continuous structure that extends through boththe rigid printed circuit board portion and the flexible printed circuitboard portion. The second portion of the inner core structure comprisesinner core circuitry In some embodiments, the second portion of theinner core structure further comprises an exposed coverlay materialcovering the inner core circuitry. In some embodiments, the regular flowprepreg layers each comprise prepreg having resin flow greater thanabout 100 mil. In some embodiments, an exposed lateral surface of therigid printed circuit board forms a rigid-flexible boundary, wherein therigid-flexible boundary formed at the exposed lateral surface issubstantially smooth and regular.

BRIEF DESCRIPTION OF THE DRAWINGS

Several example embodiments are described with reference to thedrawings, wherein like components are provided with like referencenumerals. The example embodiments are intended to illustrate, but not tolimit, the invention. The drawings include the following figures:

FIG. 1 illustrates a perspective top view of various layers included ina printed circuit board prior to stacking and lamination according tosome embodiments.

FIG. 2 illustrates an exemplary PCB stack-up 24 according to someembodiments.

FIG. 3 illustrates a cut out side view of a portion of the PCB-stack-upshown in FIG. 2 as a lamination step is performed.

FIG. 4 illustrates a cut out side view of the PCB stack-up of FIG. 3after lamination.

FIGS. 5-14 illustrate various steps in the process used to manufacture aprinted circuit board according to some embodiments.

FIGS. 15-23 illustrate various steps in the process used to manufacturea printed circuit board according to other embodiments.

FIG. 24 illustrates an exemplary PCB set form according to anembodiment.

FIG. 25 illustrates a perspective side view of the PCB set form 30 ofFIG. 24.

DETAILED DESCRIPTION OF THE EMBODIMENTS

Embodiments of the present application are directed to a printed circuitboard. Those of ordinary skill in the art will realize that thefollowing detailed description of the printed circuit board isillustrative only and is not intended to be in any way limiting. Otherembodiments of the printed circuit board will readily suggest themselvesto such skilled persons having the benefit of this disclosure.

Reference will now be made in detail to implementations of the printedcircuit board as illustrated in the accompanying drawings. The samereference indicators will be used throughout the drawings and thefollowing detailed description to refer to the same or like parts. Inthe interest of clarity, not all of the routine features of theimplementations described herein are shown and described. It will, ofcourse, be appreciated that in the development of any such actualimplementation, numerous implementation-specific decisions must be madein order to achieve the developer's specific goals, such as compliancewith application and business related constraints, and that thesespecific goals will vary from one implementation to another and from onedeveloper to another. Moreover, it will be appreciated that such adevelopment effort might be complex and time-consuming, but wouldnevertheless be a routine undertaking of engineering for those ofordinary skill in the art having the benefit of this disclosure.

FIG. 1 illustrates a perspective top view of various layers included ina printed circuit board prior to stacking and lamination according tosome embodiments. An inner core 2 includes multiple layers (not shown).In some embodiments, the inner core 2 is FCCL (flexible copper cladlaminate) or other non-conductive base material layer having aconductive layer on each surface of the non-conductive layer. In someembodiments, FCCL is made of non-conductive polyimide material withcopper on single or double sides. Polyimide is bendable. It isunderstood that alternative inner core structures can be used whichinclude a conductive layer on only one surface of the non-conductivelayer. The conductive layers are patterned and etched to form conductiveinterconnects. Select portions of the conductive interconnects, referredto as inner core circuitry, are to be part of the flexible PCB portion.Each inner core circuit is covered by a coverlay material, or simply“coverlay”. Coverlay is bendable. In some embodiments, coverlay is acombination of polyimide and adhesive. For example, coverlay is notglass reinforced like prepreg.

The dummy core 6 protects the select inner core circuitry covered by theapplied coverlay. In some embodiments, the dummy core 6 is a two-layerstructure. A first layer is a non-conductive layer, such as a basematerial. The second layer is a conductive layer, such as a copper foil.The dummy core 6 is shaped similar to an inverted stencil where thestencil pattern is formed of the dummy core material and the areasurrounding the pattern is free of material. The pattern of the dummycore 6 includes overlay portions 8 that have substantially the sameshape and size as the areas of applied coverlay 4. The pattern of thedummy core 6 also includes interconnect portions 10 that connect theoverlay portions 8 and an outer perimeter portion 12. The interconnectportions 10 and the outer perimeter portion 12 of the dummy core patternprovide a stable framework for accurately placing the overlay portions 8relative to the coverlay 4.

A layer 14 is a non-conductive, insulating layer, such as prepreg. Theprepreg used herein is a regular flow prepreg, which enables a regularpressure to be used during a subsequent lamination step, describedabove. In the PCB industry, “low flow” prepreg, such as that describedin the background, is a general term to describe prepreg with lowerresin flow than “regular flow” prepreg. “Low flow” prepreg usually hasresin flow that is less than 100 mil. “Regular flow” prepreg has resinflow that is greater than 100 mil. A layer 16 is a conductive layer,such as copper foil or laminate, where a laminate includes anon-conductive layer such as base material and a conductive layer on oneor both sides of non-conductive layer. In some embodiments, the layer 16is representative of a multilayer buildup that can include manyinterspersed conductive and non-conductive layers.

A PCB stack-up is formed by stacking various combinations of the layers,or similar to the layers, shown in FIG. 1. FIG. 2 illustrates anexemplary PCB stack-up 24 according to some embodiments. The stack-up 24includes the inner core 2, the dummy core pattern 6, the non-conductivelayer 14, the conductive layer 16, a dummy core pattern 18, anon-conductive layer 20 and a conductive layer 22. The dummy corepattern 18 can be patterned the same as the dummy core pattern 6 ordifferently depending on the inner core circuitry and coverlay patternsapplied on a back side (not shown) of the inner core 2. Thenon-conductive layer 20 can be similar to the non-conductive layer 14.The conductive layer 22 can have the same or different patternedinterconnects as the conductive layer 16. The conductive layer 16 canrepresent a single layer or a multilayer buildup, and the conductivelayer 22 can represent a single layer or multilayer buildupindependently configured than the conductive layer 16.

A laminated stack is formed by laminating the PCB stack-up shown in FIG.2. Any conventional lamination technique can be used. FIG. 3 illustratesa cut out side view of a portion of the PCB-stack-up shown in FIG. 2 asa lamination step is performed. The portion of the PCB stack-up shown inFIG. 3 coincides with an overly portion 8 of the dummy core pattern 6and a coverlay portion 4 applied over inner core circuitry of the innercore 2. An overlay portion 8′ of the dummy core pattern 18 of FIG. 2 anda coverlay portion 4′ applied over backside inner core circuitry of theinner core 2 is also shown. With the dummy core positioned on thecoverlay, the dummy core touches firmly with the coverlay under standardlamination pressure. As used herein, standard lamination pressure refersto the lamination pressure used with “regular flow” prepreg. With“regular flow” prepreg, lamination pressure is less than about 450 psi.With “low flow” prepreg, lamination pressure is more than about 450 psi.A total thickness of the dummy core and coverlay is thicker than anadjacent area such that a uniform pressure applied across the entire topand bottom surfaces of the PCB stack-up results in a higher relativepressure applied at the areas corresponding to the dummy core. Prepregresin flows into the adjacent area under lower pressure. Prepreg resinflow into the coverlay is restricted by the dummy core as well as thehigher relative pressure. The coverlay and the dummy core providestructural support during the lamination step so as to provideprotection to the inner core circuitry. FIG. 4 illustrates a cut outside view of the PCB stack-up of FIG. 3 after lamination.

FIGS. 5-14 illustrate various steps in the process used to manufacture aprinted circuit board according to some embodiments. The printed circuitboard manufactured using the various steps shown in FIGS. 5-14 issimilar to and shares features of the printed circuit boards andconstituent layers shown in FIGS. 1-4. Each of the FIGS. 5-14 illustratea cut out side view of the printed circuit board according to thevarious process steps. In FIG. 5, an exemplary inner core structure isshown. The inner core structure is a metal clad structure including abendable non-conductive layer 102 and conductive layers 104, 106 formedon both opposing surfaces. It is understood that an alternative innercore structure can be used which includes a conductive layer on only onesurface of the non-conductive layer. In some embodiments, the inner corestructure is an FCCL.

In FIG. 6, the conductive layers 104 and 106 are selectively patternetched to form inner core circuitry 108 and 110, respectively.Alternatively, the conductive layers 104, 106 are already pattern etchedduring fabrication of the inner core structure in FIG. 5. It isunderstood that FIG. 5-14 only show a portion of the printed circuitboard and in particular only show a portion of the inner core structure.Additional interconnects and circuitry may be formed on portions of theinner core structure not shown in FIGS. 5-14, those portions to beincluded as part of a rigid PCB portion of the printed circuit board.

In FIG. 7, coverlay 112, 114 is applied on the inner core circuitry 108and 110, respectively. The resulting structure forms an inner coreassembly wherein the inner core circuitry is encapsulated by thecoverlay. The coverlay has a low adhesion to a conductive layer, such ascopper, of a dummy core as described in detail below.

In FIG. 8, additional core structures and dummy core structures arefabricated, and the core structures, the inner core assembly and thedummy core structures are stacked with intervening non-conductivelayers, such as regular flow prepreg. The additional core structures canbe similar to the inner core structure of FIG. 5 with the conductivelayers pattern etched accordingly. However, the conductive layers of theadditional core structures are formed such that the resultinginterconnects will be positioned in a rigid PCB portion of the resultingprinted circuit board. In most instances, the additional core structuresare made using a non-conductive base material as opposed to FCCL. In theexemplary configuration shown in FIG. 8, two additional core structuresare included. A first core structure 122 includes a non-conductive layer124 and conductive layers 126 and 128. The conductive layers 126 and 128are selectively pattern etched. As shown in FIG. 8, the portions of theconductive layers 126 and 128 aligned with the inner core circuitry 108are removed. However, removal of the conductive layers 126 and 128 isoptional and in other embodiments these portions of the conductivelayers 126 and 128 may remain. A second core structure 130 includes anon-conductive layer 132 and conductive layers 134 and 136. Theconductive layers 134 and 136 are selectively pattern etched. As shownin FIG. 8, the portions of the conductive layers 134 and 136 alignedwith the inner core circuitry 110 are removed. However, removal of theconductive layers 134 and 136 is optional and in other embodiments theseportions of the conductive layers 134 and 136 may remain.

A dummy core 120 is positioned on the coverlay 112 of the inner coreassembly and a dummy core 121 is positioned on the coverlay 114 of theinner core assembly. The dummy core 120 includes a conductive layer 118and a non-conductive layer 116, and the dummy core 120 is oriented suchthat the conductive layer 118 is positioned against the coverlay 112.The type of coverlay used has a low adhesion to the material type of theconductive layer 118. This low adhesion enables removal of the dummycore 120 from the inner core assembly during a subsequent decap stepshown and described in relation to FIG. 13. The dummy core 121 includesa conductive layer 119 and a non-conductive layer 117, and the dummycore 121 is oriented such that the conductive layer 119 is positionedagainst the coverlay 114. The type of coverlay used has a low adhesionto the material type of the conductive layer 119. This low adhesionenables removal of the dummy core 121 from the inner core assemblyduring the subsequent decap step.

An intervening non-conductive layer 140, such as regular flow prepreg,is positioned between the dummy core 120 and the core structure 122, andan intervening non-conductive layer 142, such as regular flow prepreg,is positioned between the dummy core 121 and the core structure 130. Inthe exemplary configuration shown in FIG. 8, additional conductive layer146 and intervening non-conductive layer 138, such as regular flowprepreg, is added to the top of the stack and additional conductivelayer 148 and intervening non-conductive layer 144, such as regular flowprepreg, is added to the bottom of the stack, where the terms top andbottom are used only in relation to the orientation shown in FIG. 8. Asingle lamination step results in the laminated stack shown in FIG. 8.

In FIG. 9, selective holes are drilled through the laminated stack ofFIG. 8 to form vias, such as via 150. Vias are formed in those portionsof the printed circuit board that will be rigid PCB portions.

In FIG. 10, a desmear process is performed to remove residue, such asresidual particles from the drilling of via 150. Next, an electrolessplating process is performed to form plating 152 on the side walls ofthe via 150. In some embodiments, copper is used as the platingmaterial. It is understood that other plating materials can be used. Theplating 152 forms an interconnect with various conductive layers in thestack.

In FIG. 11, an outer conducting layer etching process is performed. Theadditional conductive layers 146 and 148 on the top and bottom,respectively, of the laminated stack are pattern etched to formpatterned conductive layers 146′ and 148′. In particular, the portionsof the conductive layers 146 and 148 aligned with the dummy cores 120and 121, respectively, are removed.

In FIG. 12, a depth controlled rout step is performed. In someembodiments, a routing tool having a rout bit is used to form a routinto the laminated stack to a depth of the conductive layer on therespective dummy core. As shown in FIG. 12, a rout 154 is made from thenon-conductive layer 138 to the conductive layer 118 of the dummy core120, and a rout 155 is made from the non-conductive layer 144 to theconductive layer 119 of the dummy core 121. FIG. 12 shows a twodimensional view of the rout 154 and 155. In three-dimensions, the routs154 and 155 are formed at an outer perimeter of the dummy cores 120 and121, respectively. A lateral rout is also performed such that theconductive layers 118 and 119 are free from surrounding prepreg material

In FIG. 13, a plug 156 is removed and a plug 157 is removed, therebyexposing the coverlay 112 and 114, respectively. The plug 156 is thearea within the rout 154 perimeter and between the non-conducive layer138 and the conductive layer 118 of the dummy core 120. The plug 157 isthe area within the rout 155 perimeter and between the non-conductivelayer 144 and the conductive layer 119 of the dummy core 121. Removal ofthe plugs 156 and 157 is referred to as a decap process. The lowadhesion between the conductive layer 118 and the coverlay 112, andbetween the conductive layer 119 and the coverlay 114 enables the plugsto simply be pulled apart from the coverlay.

FIG. 14 shows the resulting printed circuit board after the decapprocess. The exposed coverlay 112, 114, inner core circuitry 108, 110and corresponding non-conductive layer 102 form a flexible PCB portion164. Remaining portions of the laminated stack form rigid PCB portions160 and 162 on either end of the flexible PCB portion 164.

It is understood that the various structural configurations and theposition of the inner core assembly shown in the embodiments of FIGS.5-14 can be interchanged according to a specific application andapplication requirement.

FIGS. 2-14 show an exemplary configuration where both sides of the innercore circuitry are protected using coverlay and dummy core, andsubsequently exposed. This is referred to as a double-sidedconfiguration In other embodiments, the inner core circuitry is formedfrom an inner core structure that is positioned as the outer most layersof a PCB stack-up. In the case where both sides of the inner corestructure have inner core circuitry, only the inward facing side need byprotected using coverlay and a dummy core, the outward facing side isleft uncovered by coverlay. In the case where the inward facing side ofthe inner core structure does not include inner core circuitry, coverlayneed not be used and only a dummy core is used to form the flexible PCBportion. Either of these cases is referred to as a single-sidedconfiguration. FIGS. 15-23 illustrate various steps in the process usedto manufacture a printed circuit board according to other embodiments.The printed circuit board manufactured using the various steps shown inFIGS. 15-23 is similar to and shares features of the printed circuitboard and constituent layers shown in FIGS. 5-14 except that the FIGS.15-23 are directed to a single-sided process where the inward facingside of the inner core structure doe not include inner core circuitry.Each of the FIGS. 15-23 illustrate a cut out side view of the printedcircuit board according to the various process steps.

In FIG. 15, an exemplary inner core structure is shown. The inner corestructure is a metal clad structure including a non-conductive layer 202and conductive layers 204 and 206 formed on both opposing surfaces. Itis understood that an alternative inner core structure can be used whichincludes a conductive layer on only one surface of the non-conductivelayer. In some embodiments, the inner core structure is an FCCL.

In FIG. 16, the conductive layer 206 is selectively pattern etched toform pattern etched conductive layer 208 and at least to expose surface203 of the non-conductive layer 202. Alternatively, the conductive layer206 is already pattern etched during fabrication of the inner corestructure in FIG. 15. It is understood that FIG. 15-23 only show aportion of the printed circuit board and in particular only show aportion of the pattern etched layer 208. Additional interconnects andcircuitry may be formed on portions of the inner core structure notshown in FIGS. 15-23, those portions to be included as part of a rigidPCB portion of the printed circuit board.

In FIG. 17, additional core structures and a dummy core structure arefabricated, and the core structures, the inner core structure of FIG. 16and the dummy core structure are stacked with intervening non-conductivelayers, such as regular flow prepreg. The additional core structures canbe similar to the inner core structure of FIG. 15 with the conductivelayers pattern etched accordingly. However, the conductive layers of theadditional core structures positioned are formed such that the resultinginterconnects will be positioned in a rigid PCB portion of the resultingprinted circuit board. In those additional core structures that includeportions of a subsequent plug to be removed during a decap process, thecorresponding conductive layer portions can be etched away or leftintact. In most instances, the additional core structures are made usinga non-conductive base material as opposed to FCCL. In the exemplaryconfiguration shown in FIG. 17, two additional core structures areincluded. A first core structure 222 includes a non-conductive layer 224and conductive layers 226 and 228. The conductive layers 226 and 228 areselectively pattern etched. As shown in FIG. 17, the portions of theconductive layers 226 and 228 aligned with exposed surface 203 areremoved. Alternatively, the portions of the conductive layers 226 and228 aligned with the exposed surface 203 can be left intact. A secondcore structure 230 includes a non-conductive layer 232 and conductivelayers 234 and 236. The conductive layers 234 and 236 are selectivelypattern etched. As shown in FIG. 17, the portions of the conductivelayers 234 and 236 aligned with the exposed surface 203 may includepatterned interconnects.

A dummy core 220 is positioned on the exposed surface 203 of the innercore structure. The dummy core 220 includes a conductive layer 218 and anon-conductive layer 216. The dummy core 220 is oriented such that theconductive layer 218 is positioned against the exposed surface 203. Thebase material of the non-conductive layer 202 has a low adhesion to thematerial type of the conductive layer 218. This low adhesion enablesremoval of the dummy core from the inner core structure during asubsequent decap step shown and described in relation to FIG. 22.

An intervening non-conductive layer 238, such as regular flow prepreg,is positioned between the dummy core 220 and the core structure 222, andan intervening non-conductive layer 242, such a s regular flow prepreg,is positioned between the core structure 222 and the core structure 230.

In FIG. 18, selective holes are drilled through the laminated stack ofFIG. 17 to form vias, such as via 250. Vias are formed in those portionsof the printed circuit board that will be rigid PCB portions.

In FIG. 19, a desmear process is performed to remove residue, such asresidual particles from the drilling of via 250. Next, an electrolessplating process is performed to form plating 252 on the side walls ofthe via 250. In some embodiments, copper is used as the platingmaterial. It is understood that other plating materials can be used. Theplating 252 forms an interconnect with various conductive layers in thestack.

In FIG. 20, an outer conducting layer etching process is performed. Theoutward facing conductive layer 204 of the inner core structure ispattern etched to form inner core circuitry 210 and conductive layer 236is pattern etched to form patterned conductive layers 236′. Inparticular, the portion of the conductive layers 246 aligned with thedummy core 220 is removed.

In FIG. 21, a depth controlled rout step is performed. As shown in FIG.21, a rout 254 is made from the non-conductive layer 232 to theconductive layer 218 of the dummy core 220. FIG. 21 shows a twodimensional view of the rout 254. In three-dimensions, the rout 254 isformed at an outer perimeter of the dummy core 220. A lateral rout isalso performed such that the conductive layer 218 is free fromsurrounding prepreg material.

In FIG. 22, a plug 256 is removed, thereby exposing the surface 203 ofthe non-conductive layer 202. The plug 256 is the area within the rout254 perimeter and between the non-conducive layer 232 and the conductivelayer 218 of the dummy core 220. The low adhesion between the conductivelayer 218 and the surface 203 enables the plug to simply be pulled apartfrom the non-conductive layer 202.

FIG. 23 shows the resulting printed circuit board after the decapprocess. The inner core circuitry 210 and the portion of thenon-conductive layer 202 corresponding to the exposed surface 203 form aflexible PCB portion 264. Remaining portions of the laminated stack formrigid PCB portions 260 and 262 on either end of the flexible PCB portion264.

It is understood that the various structural configurations shown in theembodiments of FIGS. 15-23 can be interchanged according to a specificapplications and application requirement.

In some manufacturing processes, multiple PCBs are manufactured asdiscrete portions of a single substrate, which are separated intoindividual PCBs at the end of the manufacturing process. Such a singlesubstrate configuration is referred to as a PCB set form. FIG. 24illustrates an exemplary PCB set form according to an embodiment. Theexemplary PCB set form 30 includes two PCBs. A first PCB includes arigid PCB portion 32 and a flexible PCB portion 36. A second PCBincludes a rigid PCB portion 34 and a flexible PCB portion 38. Incontrast to the PCBs shown in FIGS. 14 and 23 that include two rigid PCBportions per PCB, the PCBs shown in FIG. 24 each include only a singlerigid PCB portion. It is understood that PCB set forms can include moreor less than the exemplary two PCBs shown in FIG. 24. The two PCBs areconnected physically but not electrically. In some embodiments, anadditional routing step is applied to an outer perimeter portion of aPCB set form. The additional routing step can be performed at any pointin the printed circuit board manufacturing process after the laminationstep is performed. For example, in the printed circuit boardmanufacturing process shown in FIGS. 5-14, the additional routing stepcan be performed at any point after the lamination step shown in FIG. 8.The additional routing step removes a perimeter portion of the laminatedPCB stack up including the outer perimeter portion of the dummy corepattern, such as the outer perimeter portion 12 of FIG. 1. A resultingperimeter area surrounding the PCBs in the PCB set form 30 is shown inFIG. 24 as breakaway area 46.

The rigid PCB portions 32, 34 are ready for surface components to bemounted on select areas, such as through a surface mount technology(SMT) process. After the components are mounted, the PCBs are separatedfor subsequent installation into other devices. Separating the PCBs canbe performed using any conventional process including, but not limitedto, cutting the PCB set form 30 along etched lines. Cutting along theperimeter etch lines separates the breakaway area 46 from the PCBs.

FIG. 25 illustrates a perspective side view of the PCB set form 30 ofFIG. 24. The breakaway area 46 includes a dummy core portion 60, whichis a remnant of a dummy core pattern used to protect the flexible PCBportion 38 of FIG. 24. As exemplified in FIG. 1, a dummy core patterncan include an interconnect portion, such as the interconnect portion 10of the dummy core pattern 6 in FIG. 1, a portion of which coincides withthe breakaway area of a PCB set form, such as the breakaway area 46 inFIG. 25. As such, although the outer perimeter portion of the dummy corepattern, such as the outer perimeter portion 12 in FIG. 1, as well asthe overlay portions of the dummy core pattern that are applied over thecoverlay and corresponding inner core circuitry, such as the overlayportions 8 in FIG. 1, are removed during the PCB manufacturing process,the interconnect portions that connect to the outer perimeter portion ofthe dummy core pattern remain.

In some embodiments, the flexible PCB portions can be formed asconnector sections between rigid PCB portions, such as the configurationshown in FIG. 24. The flexible PCB portion is flexible thereby enablingtwo adjoining rigid PCB portions to rotate, or pivot, relative to eachother.

The printed circuit board and manufacturing processes described hereinprovided numerous advantages. The printed circuit board having bothrigid PCB portions and a flexible PCB portions is formed using regularflow prepreg. In prior art printed circuit boards, flexible PCB portionsare formed using low flow prepreg as well as lamination accessories suchas release film and conformal film. Use of low flow prepreg is needed tocontrol squeeze out during lamination. However, since low flow prepregis used, a greater lamination pressure is required which results insurface ripple on the PCB exterior surfaces. Under high pressure theunderlying topography of the inner layer circuitry is reflected on thesurface resulting in the irregular, or rippled, surface. In the presentapplication, there is no need to control resin squeeze out, there is nolimitation in prepreg selection, there is no need of laminationaccessories or high lamination pressure, which results in a flatexterior surfaces. The present process improves board flatness thatsolves impedance control issues and improves reliability of surfacemounted component connections. Yield of fine line 2/2 mil etching andsoldermask fine line imaging is also improved because of the flatexterior surfaces. Without use of lamination accessories and yieldimprovement, the process of the present application saves running costdramatically. Higher pressure lamination as used in conventionalprocesses leads to expansion in the X-Y plane of the PCB. Such lateralexpansion moves surface contact pads relative to their designedpositions. The present process uses standard lamination pressure andtherefore reduces lateral expansion. Such dimensional control isbecoming more and more significant with smaller and smaller pitchcomponents to be surface mounted.

The printed circuit board and manufacturing process described hereinalso resolves the resin squeeze out issue at the rigid-flex boundary. Inthe present process, a well controlled and regular rigid-flex boundaryis achieved while the prior art processes have poor control andirregular rigid-flex boundary which varies lot by lot. Conventionalprocesses using low flow prepreg result in rougher, more irregularrigid-flex boundary. Such an irregular boundary affects reliability ofthe flexible PCB portion. In the current application, resin flow isrestricted by the dummy core and a rigid-flexible boundary is defined bydepth control rout. Therefore, the rigid-flexible boundary issubstantially smooth and regular.

The present process also enables precise removal of the plug using thedescribed decap process steps.

Standard rigid PCB design can be transferred to rigid-flex designsmoothly using the present process, this expands product categories suchas HDI, ELIC, 0.3 mm BGA pitch and sequential lamination, and henceincreases business opportunities.

An advantage of using the coverlay and dummy core in the manufacturingprocess is that relatively early in the manufacturing process a finalcircuit surface, for example the inner core circuitry, can be preparedand protected during subsequent process steps. The final covered circuitsurface can be re-exposed from other layers of a PCB laminated stacklater in the process without having been contaminated.

The present application has been described in terms of specificembodiments incorporating details to facilitate the understanding of theprinciples of construction and operation of the printed circuit board.Many of the components shown and described in the various figures can beinterchanged to achieve the results necessary, and this descriptionshould be read to encompass such interchange as well. As such,references herein to specific embodiments and details thereof are notintended to limit the scope of the claims appended hereto. It will beapparent to those skilled in the art that modifications can be made tothe embodiments chosen for illustration without departing from thespirit and scope of the application.

1. A printed circuit board comprising: a. a rigid printed circuit boardportion comprising a laminated stack of a plurality of non-conductinglayers and a plurality of conductive layers, wherein the laminated stackfurther comprises a first portion of an inner core structure; and b. aflexible printed circuit board portion comprising a second portion ofthe inner core structure, wherein the inner core structure is acontinuous structure that extends through both the rigid printed circuitboard portion and the flexible printed circuit board portion, furtherwherein the second portion of the inner core structure comprises innercore circuitry and exposed coverlay material covering the inner corecircuitry.
 2. The printed circuit board of claim 1 wherein each of theconductive layers is pattern etched.
 3. The printed circuit board ofclaim 1 further comprising one or more plated through hole vias in therigid printed circuit board portion.
 4. The printed circuit board ofclaim 1 wherein the rigid printed circuit board portion comprises afirst rigid printed circuit board portion, further wherein the printedcircuit board further comprises a second rigid printed circuit boardportion comprising a second laminated stack of a plurality ofnon-conducting layers and a plurality of conductive layers, wherein thesecond laminated stack further comprises a third portion of the innercore structure, further wherein the flexible printed circuit boardportion is coupled between the first rigid printed circuit board portionand the second rigid printed circuit board portion.
 5. The printedcircuit board of claim 1 wherein the inner core structure comprises aninner core non-conductive layer having a first surface and a firstconductive layer positioned on the first surface of the inner corenon-conductive layer.
 6. The printed circuit board of claim 5 whereinthe first conductive layer of the inner core structure comprises theinner core circuitry in the second portion of the inner core structure.7. The printed circuit board of claim 6 wherein the inner corenon-conductive layer has a second surface opposing the first surface,further wherein the inner core structure further comprises a secondconductive layer positioned on the second surface of the inner corenon-conductive layer.
 8. The printed circuit board of claim 7 whereinthe second conductive layer of the inner core structure comprises theinner core circuitry in the second portion of the inner core structure.9. The printed circuit board of claim 5 wherein the inner corenon-conductive layer comprises polyimide.
 10. The printed circuit boardof claim 1 wherein the coverlay material comprises a combination ofpolyimide and adhesive.
 11. A printed circuit board set form comprising:a. a plurality of printed circuit boards aligned within a common plane,wherein each printed circuit board is mechanically connected by a commonsubstrate, further wherein each printed circuit board comprises: i. arigid printed circuit board portion comprising a laminated stack of aplurality of non-conducting layers and a plurality of conductive layers,wherein the laminated stack further comprises a first portion of aninner core structure; and ii. a flexible printed circuit board portioncomprising a second portion of the inner core structure, wherein theinner core structure is a continuous structure that extends through boththe rigid printed circuit board portion and the flexible printed circuitboard portion, further wherein the second portion of the inner corestructure comprises inner core circuitry and exposed coverlay materialcovering the inner core circuitry; and b. a breakaway substrate alignedwithin the common plane and mechanically connected around a perimeter ofthe connected plurality of printed circuit boards, wherein the breakawaysubstrate includes a dummy core portion.
 12. The printed circuit boardset form of claim 9 wherein the breakaway substrate provides lateralstructural stability to the connected plurality of printed circuitboards.
 13. The printed circuit board set form of claim 9 wherein theplurality of printed circuit boards are electrically isolated from eachother.
 14. A method of manufacturing a printed circuit board comprising:a. forming an inner core structure having an inner core circuitry on atleast one surface of the inner core structure; b. applying a coverlaymaterial over the inner core circuitry; c. forming a printed circuitboard stack up, wherein the printed circuit board stack up comprises theinner core structure, a dummy core, one or more non-conductive layersand one or more conductive layers, wherein the dummy core is stacked onthe coverlay material; d. laminating the printed circuit board stack up,thereby forming a laminated stack; e. forming a depth controlled routfrom a surface of the laminated stack to the dummy core and around aperimeter of the dummy core, wherein a portion of the laminated stackwithin the perimeter of the rout and to a depth including the dummy coreforms a laminated stack cap; f. removing the laminated stack cap,thereby exposing the coverlay material and forming a flexible portion ofthe printed circuit board.
 15. The method of claim 14 wherein theperimeter of the dummy core corresponds to a perimeter of the inner corecircuitry.
 16. The method of claim 14 further comprising forming thedummy core, wherein the dummy core comprises a non-conductive layer anda conductive layer.
 17. The method of claim 16 wherein the dummy core isstacked on the coverlay material such that the conductive layer of thedummy core contacts the coverlay material.
 18. The method of claim 14further comprising forming at least one plated through hole via in thelaminated stack prior to forming the depth controlled rout, wherein theat least one plated through hole via is not aligned within the innercore circuitry.
 19. The method of claim 14 further comprising patternetching the conductive layers in the laminated stack prior to formingthe printed circuit board stack up.
 20. The method of claim 14 whereinforming the inner core structure comprises applying a first conductivelayer on a first surface of a non-conductive layer and applying a secondconductive layer on a second surface of the non-conductive layer. 21.The method of claim 20 wherein the first conductive layer is patternetched and the second conductive layer is pattern etched.
 22. The methodof claim 14 wherein the one or more non-conductive layers comprise oneor more regular flow prepreg layers.
 23. The method of claim 22 whereinlaminating the printed circuit board stack up comprises applying astandard lamination pressure less than about 450 psi.
 24. The method ofclaim 23 wherein a remaining portion of the laminated stack outside theperimeter of the rout forms a rigid portion of the printed circuitboard, wherein an exposed outer surface of the laminated stack is smoothand non-rippled due to laminating the printed circuit board stack upusing regular lamination pressure and the inclusion of regular flowprepreg.
 25. A printed circuit board comprising: a. a rigid printedcircuit board portion comprising a laminated stack of a plurality ofnon-conducting layers and a plurality of conductive layers, wherein theplurality of non-conducting layers comprises a plurality of regular flowprepreg layers, further wherein an exposed outer surface of thelaminated stack is smooth and non-rippled, wherein the laminated stackfurther comprises a first portion of an inner core structure; and b. aflexible printed circuit board portion comprising a second portion ofthe inner core structure, wherein the inner core structure is acontinuous structure that extends through both the rigid printed circuitboard portion and the flexible printed circuit board portion, furtherwherein the second portion of the inner core structure comprises innercore circuitry
 26. The printed circuit board of claim 25 wherein thesecond portion of the inner core structure further comprises an exposedcoverlay material covering the inner core circuitry.
 27. The printedcircuit board of claim 25 wherein the regular flow prepreg layers eachcomprise prepreg having resin flow greater than about 100 mil.
 28. Theprinted circuit board of claim 25 wherein an exposed lateral surface ofthe rigid printed circuit board forms a rigid-flexible boundary, whereinthe rigid-flexible boundary formed at the exposed lateral surface issubstantially smooth and regular.